The present invention relates to an improved technique for forming predetermined electrical circuitry patterns on dielectric substrates, particularly ceramic substrates.
Circuit "boards" or "cards" carrying a vast array of different electronic components are widely used in many different types of electrical devices such as computers and the like. One type of electronic component typically carried on a circuit board is a "module" comprising a ceramic substrate carrying one or more "chips" or other electrical devices. The ceramic substrates of such modules typically carry on one or both major surfaces thereof electrical circuitry patterns for electrical contact with the chips and discrete features for other elements mounted thereon. In the industry, it is typical to refer to modules comprising ceramic substrates having one or more chips thereon as "first level packages" and to refer to the circuit boards or cards on which the first level packages or modules are mounted as "second level packages."
The electrical circuitry patterns carried on the ceramic substrates of conventional first level packages or modules typically are composed of discrete, electrically conducting metal pathways and non-conducting spaces therebetween. In manufacture, they are formed by a complicated procedure which involves a "first pass" or "personalization" for forming the predetermined electrical circuitry pattern usually by photoresist process and a "second pass" or "select pass" for treating selected portions of the metal pathways formed in the first pass to promote solder adhesion thereto.
For example, in a typical photoresist process for this purpose, the first pass usually involves depositing a continuous metal layer on the ceramic substrate, forming an adherent photoresist mask on the metal layer defining a pattern of openings corresponding to the non-conducting spaces in the target electrical circuitry pattern, etching to remove the metal exposed by these openings and finally removing the resist to thereby produce the first pass product. In the second pass, a procedure essentially identical to the first pass process is used, except that the pattern of the photoresist is changed so that only the portions of the metal pathways which are to be treated for solder adhesion are exposed. In addition, the chemicals are changed so that etching merely promotes solder adhesion rather than completely removes the metal.
Photoresist technology for producing predetermined electrical circuitry patterns on dielectric substrates, such as ceramic substrates, has developed to the point where extremely high "circuit densities" can be produced. For example, processes available today can produce electrical circuitry patterns whose metal pathways are as thin as one mil (0.001") and whose non-conducting spaces therebetween are as thin as two mils (0.002").
Although many applications require the production of electrical circuitry patterns or discrete features with very high circuit densities (i.e., very thin metal pathways and very thin non-conducting spaces therebetween), many other applications do not. For example, in many applications, circuit densities having metal pathway widths on the order of 3 to 4 mils or more and space widths of 4 to 5 mils or more are entirely acceptable.
Typical photoresist processes, as described above, are relatively expensive to carry out. This is because they involve many process steps and typically employ many different chemicals, some of which are environmentally dangerous. Accordingly, there is a need for a technique for producing electrical circuitry patterns on dielectric substrates, particularly ceramic substrates, which is less complicated and preferably uses fewer chemicals than prior art processes. Such a technique would be particularly advantageous in those applications in which the high circuit densities made possible by modern photoresist technologies were unnecessary.
Accordingly, it is an object of the present invention to provide a new technique for forming electrical circuitry patterns and discrete features on dielectric substrates, particularly ceramic substrates, which is less complicated, less expensive and uses fewer chemicals than conventional photoresist processes practiced currently.